Overvoltages occur every now and then and they cause damage in sensitive I/O electronics. I/O voltages are of the order of 0 . . . 32 V. For example, if a mains voltage (230 V, 50 Hz) happens to be applied to an I/O pin, ordinary I/O electronics will be damaged beyond repair. This results in maintenance problems and increased costs. There are various components, such as varistors and overvoltge Zeners, for protection against lightning and instantaneous overvoltages. However, if a voltage of 230 V is supplied directly to an electronic circuit, such components will not withstand it. If an installer makes a mistake, 230 V may be connected to low-voltage circuits, and this will result in further damage to equipment and safety risks. The components cannot endure a voltage of 230 V and they have a very limited power capacity. The method of the invention prevents the application of an overvoltage to sensitive I/O electronics in the event of a malfunction. In this protection system, the voltage at an I/O pin will never exceed the allowed values. The invention provides the following advantages:
does not cause any change in the operating points of the I/O electronics or in any other delays or impedances of the I/O electronics PA1 simple I/O lines PA1 cheap when there are a large number of I/O lines PA1 allows high data transmission speeds PA1 filters radio-frequency interference PA1 requires no filters PA1 can be installed as an option in old elevators, enabling old circuits to be protected afterwards.